Method and apparatus for process and quality control in the production of metal

ABSTRACT

A method and apparatus are provided for rapidly and accurately processing the chemical composition of a molten metal bath within a furnace. The apparatus includes a robotically-controlled probe which operates to obtain a sample from any predetermined depth of the vessel. The probe is moved from the vessel to a thermostabilized atomic emission spectrometer in nearby proximity to the vessel wherein a part of the probe containing the sample is joined with an upper chamber housing to form an excitation chamber. Within the excitation chamber, the optical emission spectrum of the sample is excited by an excitation source. The optical spectrum is transmitted to an analyzer where the elemental concentration of the sample is determined. Immediately responsive to this analysis, adjustments are made in processing to achieve the target grade of steel.

TECHNICAL FIELD

This invention relates to process and quality control in themetallurgical industry, and in particular to a method and apparatus forthe direct analysis of molten metal composition during production usingatomic emission spectrometry.

BACKGROUND ART

Atomic emission spectroscopy is an analytical method which is used fordetermining and measuring the chemical composition of materials.According to this method, a material sample is heated and vaporized by aspectral excitation source. Atoms of the sample are excited andpartially ionized, causing them to emit light in the ultraviolet,visible, or near infrared spectral region. The radiated light ischaracterized by an arrangement of spectral lines, and the intensity ofthese spectral lines indicates the atomic concentration within thesample.

Atomic emission spectroscopy has been used to determine materialcomposition during steel making, since the physical properties of ametal, such as strength, hardness, and corrosion resistance, depend inpart on its composition. Improving production efficiency has beendifficult because analysis of the chemical composition of molten metalwithin a furnace has not been rapid and accurate enough to change theproduction process of the melt being analyzed. Furthermore, the hot,dirty environment of a furnace limits the type of apparatus that can beused in close proximity for sampling and testing to detect the presenceof the constituent elements.

There have been several different methods used to determine thecomposition of molten metal within a furnace. Certain prior art devicespump molten metal to a remote analytical laboratory. This configurationobviates the need for environmentally protecting the sophisticatedoptical instrumentation, but results in very long analysis times andhigh construction costs. Other devices obtain samples from near the meltsurface, where the slag layer can interfere with the analysis. Yet otherdevices utilize a probe inserted into the melt to excite a small portionof the molten metal while still in the furnace. While such a methodavoids costs associated with sampling, high development and materialscosts are required to position and operate optical instrumentationwithin the probe in such an extreme temperature environment.

DISCLOSURE OF THE INVENTION

The present invention overcomes the above-mentioned disadvantages byproviding a method and apparatus for processing metal, such as duringproduction, which provides a spectral determination of the chemicalcomposition of metal in the high temperature environment proximate thefurnace, that improves the accuracy and reduces the time for elementalanalysis so that adjustments can be made during production at a lowcost.

The present invention utilizes a probe capable of obtaining a sample ofmolten metal from any predetermined depth of a vessel and depositing thesample in a nearby analyzer. The time required for the analysis isreduced through the use of a robotic system to move the probe and amethod by which a part of the probe containing the sample is depositeddirectly into a thermostabilized atomic emission spectrometer proximatethe furnace. Within the atomic emission spectrometer, the depositedprobe part is joined with an upper chamber housing to form an excitationchamber. Within the excitation chamber, the optical emission spectrum ofthe sample is excited by an excitation source. The optical spectrum istransmitted to an analyzer where the elemental concentration of thesample is determined. Immediately responsive to this analysis,adjustments are made in processing to achieve the target grade of steel.As a result of these features, alterations in the composition of themelt may be enacted more efficiently and accurately during processing,thereby shortening the overall melting duration and allowing expensivealloys to be produced more economically.

The above features and other features and advantages of the presentinvention are more readily understood from a review of the attacheddrawings and the accompanying specification and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a metal processing apparatusconstructed in accordance with the present invention;

FIG. 2 is a sectional view of a probe utilized in the apparatus of FIG.1 and its connection to a probe holder and linear traversing unit of thepresent invention;

FIG. 3 is a sectional view of the separation of the probe of the presentinvention;

FIGS. 4a, 4b, and 4c are sectional views of the removal of the probefrom the probe holder, engagement of the probe with an excitationchamber, and deposition of a lower part of the probe in the excitationchamber, respectively;

FIG. 5 is a sectional view of the excitation chamber shown in FIG. 4 inaccordance with the present invention;

FIGS. 6a and 6b are the schematic diagrams of preferred charge anddischarge circuits, respectively, of a spectral excitation source foruse in the present invention;

FIG. 7 shows a multichannel polychromator in accordance with the presentinvention; and

FIG. 8 is a schematic diagram of preferred signal measurementinstrumentation in accordance with the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Referring first to FIG. 1, a metal processing apparatus 10 in accordancewith the present invention includes a furnace 12 containing a bath 14 ofmolten metal. Metal processing apparatus 10 generally includes a probe16 for obtaining a sample of molten metal from within bath 14, a lineartraversing unit 18 for positioning probe 16 within bath 14, a doser 19for adding elements to bath 14, an atomic emission spectrometer 20 foranalyzing the sample obtained from bath 14, a robotic arm 22 fortransferring probe 16 from bath 14 to atomic emission spectrometer 20,an autosampler 24 containing a plurality of probes 16, and a centralcontroller 26 which governs all actions associated with apparatus 10.Atomic emission spectrometer 20 and its components are thermostabilized,such as with a refractory material, so that they may function in closeproximity to furnace 12.

Probe 16 is preferably disposable and constructed of a non-corrosive,high melting-point metal, such as tungsten, or a refractory material,such as fire-clay. For insertion into bath 14, robotic arm 22 fastens aprobe 16 from autosampler 24 to a three-pronged probe holder 28, asshown in FIG. 2. Probe holder 28, which is preferably composed ofgraphite, is connected to linear traversing unit 18 for submersion intobath 14. In compliance with the melting schedule or at an operator'srequest, furnace 12 is opened, linear traversing unit 18 is rotated andangled toward furnace 12, and probe holder 28 and attached probe 16 aresunk into bath 14. Probe 16 is inserted through the top layer of slag,and is selectively located at a desired depth and position within bath14 by linear traversing unit 18. After positioning, linear traversingunit 18 performs several smooth horizontal motions for quality fillingof probe 16 with molten metal.

In a preferred embodiment, probe 16 is constructed from two hermeticallysealed parts, as depicted in FIG. 3. The lower probe part 30 is providedwith inlets 32 to allow the inflow of molten metal. Inlets 32 areinitially closed by a temperature-destructive material, such as cork.With a quick passage of probe 16 across the slag, inlets 32 remainclosed such that slag material is not sampled. After probe 16 has passedacross the slag, the corks disintegrate and molten metal is allowed toenter probe 16 and fill a trough 34 provided therein. The upper probepart 36 is preferably formed with an insert 38 having a planar surface40 for forming a planar surface 42 on the sample 44.

As shown in FIG. 4, after removal from bath 14, probe 16 is controlledby robotic arm 22 in order to separate probe 16 from probe holder 28(FIG. 4a), deposit lower probe part 30 into table 47 (FIG. 4b) withinatomic emission spectrometer 20, and then separate upper probe part 36from lower probe part 30 to expose sample 44 (FIG. 4c). Since sample 44is solid or liquid in form, the elements of interest for spectralanalysis are bonded to other elements. In order to utilize atomicspectrometry, the individual bonds must be broken and sample 44converted to a gaseous, or plasma, state.

Referring now to FIG. 5, table 47 is operable to move upward so thatlower probe part 30 engages an upper chamber housing 49 via connectionpins 45 to enclose sample 44 and form an excitation chamber 46. A scandriver 48 actuated by a stepper motor (not shown) is used for preciseplacement of the planar surface 42 of sample 44 on the optical axis ofexcitation chamber 46. Upper chamber housing 49 of excitation chamber 46is provided with several high voltage rod electrodes 50 which areoperably connected to a spectral excitation source 52 (shownschematically in FIG. 6). Spectral excitation source 52 applies a highvoltage between electrodes 50, which generates electrical dischargesbetween planar sample surface 42 and electrodes 50, forming analyticalgaps 54 therebetween.

During a preliminary heating cycle, spots at sample surface 42 areheated to provide more homogeneous evaporation of sample 44, as is wellknown in the art. The temperature of the spots is controlled by thedischarge power of spectral excitation source 52. The spots are blown byan inert gas, such as argon, which is delivered from a pressurized gassource (not shown) into excitation chamber 46. As is well known in theart, the inert gas is used to provide more stable measurements,eliminate possible oxidation of sample surface 42, and provide spectraltransmission in the ultraviolet range.

The discharge between electrodes 50 and sample surface 42 erodes smallparticles from sample 44 which are excited by spectral excitation source52. In response, sample 44 emits radiation 56 in accordance with theelements in the sample 44. The characteristic frequencies of emittedradiation 56, the emission spectrum, identify the constituent elements,and the intensities of the radiation indicate the quantities thereof. Afiber optic cable 58 or optical lens is exposed to the emitted radiation56, and establishes an output signal containing the several wavelengthsof the elements in the solution which is transmitted from analyticalgaps 54 to an optical detector, preferably a multichannel polychromator60 (best shown in FIG. 7).

As is well known in the art, a spark generator provides high voltagesparks as the electrical discharge, while an arc generator provides analternating current discharge. As depicted in FIGS. 6a and 6b, spectralexcitation source 52 preferably provides a CLV discharge, meaning thatcapacitors, inductances, and nonlinear elements such as diodes areinvolved in the formation of the current impulse. A capacitor unit 62 ischarged from a power source 64 through a special charge circuit 66 (FIG.6a) and discharged through a discharge circuit 68 and analytical gaps 54(FIG. 6b). Charge circuit 66 also provides power for a rise front unit70 (FIG. 6a), and an ignition unit 72 (FIG. 6b) which are connected withanalytical gaps 54 and contain capacitors for energy storing. A controlunit 74 controls parameters of capacitor unit 62, charge circuit 66, anddischarge circuit 68.

Each discharge cycle is generated in two stages. In the first stage,capacitor unit 62, rise front unit 70, and ignition unit 72 are chargedfrom charge circuit 66 to a predetermined voltage level as controlled bycontrol unit 74. When a predefined level is reached, the charge currentis interrupted. Electromagnetic energy that is stored in inductance L ofcharge circuit 66 in the moment of current interruption returns tocapacitor C associated with power source 64.

A special feature of the present invention is that resistive elementsare not used in charge circuit 66, but only inductances and switches.This increases the effectiveness of spectral excitation source 52,prevents heat dissipation, and allows spectral excitation source 52 tobe placed in close proximity to sensitive optical instruments. Currentswitching in charge circuit 66 and discharge circuit 68 is performed bypower electronic switches, such as thyristors Q₀, Q₁, and Q₂, accordingto commands from control unit 74 which compare the voltage on capacitorsC₀, C₁, and C₂ within capacitor unit 62 with a reference level. Thetotal number of charged capacitors as well as the voltage level incapacitor unit 62 is defined by control unit 74 and may change frompulse to pulse.

After charging to determine the voltage, capacitors C₀, C₁, and C₂ aredisconnected from power source 64. In the second stage, capacitor unit62 discharges through discharge circuit thyristors Q₃ and Q₄,inductances L₁, and L₂, and analytical gaps 54. As a rule, the voltageof the capacitors within capacitor unit 62 is not high enough to performan electrical breakdown of analytical gaps 54. Therefore, ignition unit72 is used for stable, reproducible breakdown of analytical gaps 54using a high voltage (˜10 kV) and very small power spark.

Parameters of discharge circuit 68 are under control of control unit 74and may be changed remotely rather than requiring operator access toatomic emission spectrometer 20. Depending on the parameters ofdischarge circuit 68 and the total capacitance of capacitor unit 62, itis possible to get different types of discharge pulses. The dischargemay be an arc-type discharge (relatively small current and long pulseduration) or a spark-type discharge (relatively large current and smallpulse duration). Each discharge type has its benefits, and rise frontunit 70 is used with the purpose of producing a combined discharge typewhich integrates the beneficial features of both types.

At the start of the discharge process, rise front unit 70 generates aspark-type discharge in the shape of a sharp unipolar current peak(˜50-100 A/20-50 μsec) which excites the spectral lines of ions.Discharge circuit 68 generates the main part of the discharge currentpulse, which is an arc-type discharge formed in the shape of anaperiodic current impulse (˜20-100 A/250-400 μsec) which excites thespectral lines of neutral atoms. The pulse energy and wave form aredefined by capacitances, inductances, and the voltage level. Theseparameters as well as the frequency and duration of the pulse are allcontrolled by control unit 74. Spectral excitation source 52 generates arepetitive sequence of discharge pulses, and the whole charge/dischargeprocess is repeated with computer-controlled frequency.

Referring next to FIG. 7, shown is multichannel polychromator 60, whichprovides optical dispersion of the radiation 56 that sample 44 emits andseparation of the individual spectral lines. In the preferredembodiment, the classical Pashen-Runge scheme is utilized forpolychromator 60. In this scheme, light enters polychromator 60 throughfiber optic cable 58 and an entrance slit 80, the various spectralfrequencies are dispersed by a concave, reflection-type diffractiongrating 82 and then directed according to wavelength into exit slits 84which cut spectral lines of required wavelength. Entrance slit 80, exitslits 84, and the center of diffraction grating 82 are situated on theRowland circle. Preferably, diffraction grating 82 is characterized by3600 grooves/mm and a curve radius of 498.1 mm. Polychromator 60consists of an optical bench 81, an illuminating system (not shown), anda mechanism for entrance slit scanning 83.

Entrance slit 80 is installed with a micrometric screw which allowsentrance slit 80 to be moved tangentially to the Rowland circle by acomputer-controlled stepper motor (not shown) for very accurate spectralline profiling. Exit slits 84 are placed at exact locations along thefocal curve 85 to allow passage of light of those specific wavelengthsfor the elements which are to be measured. Due to certain factors, suchas climatic changes or the plastic deformation of optical bench 81, thespectrum position on the Rowland circle may vary with time. The scanningmechanism 83 of fiber optic cable 58 and entrance slit 80 is intendedfor the compensation of the spectrum shift. The second function ofscanning mechanism 83 is to check if exit slit 84 is installedcorrectly. Scanning should be directed tangentially to the Rowlandcircle at the point of entrance slit 80.

Multichannel polychromator 60 can simultaneously detect up to 15individual wavelengths, with the option of up to 24. The separatedfrequencies of radiation pass through exit slits 84, and light sensors,such as photomultiplier tubes 86, are precisely positioned behind exitslits 84 to measure light passing therethrough. Photomultiplier tubes 86detect the light emission and generate proportional electrical signals,in particular, current. Photomultiplier tubes 86 within polychromator 60amplify the various signals produced by the incoming spectra, and theamplified output signals are connected to the input of an analogmultiplexor 88 as shown in FIG. 8.

Specifically, each channel 90 within polychromator 60 consists of anactive integrator in the form of an operational amplifier 92 with acapacitor 94 in feedback, along with a switch 96 in parallel withcapacitor 94. When switch 96 is open, the input current is integratedand the results are stored in capacitor 94. When switch 96 is closed,capacitor 94 is discharged and the channel 90 is reset. The output ofmultiplexor 88 is connected to an A/D converter 98 which, in turn,transmits data to a computer 100 for storage in memory. Computer 100also controls the position of switch 96.

At the beginning of a working cycle, controlling information is loadedfrom central controller 26 to an atomic emission spectrometer controlunit 102 and a robotic control unit 104 (best shown in FIG. 1). Thisinformation defines all parameters of each discharge pulse and allparameters of robotic motions. Next, charge circuit 66 of spectralexcitation source 52 charges capacitor unit 62 and all channels 90 arereset. Then, spectral excitation source 52 initiates the discharge.Switches 96 are opened and integration of the input current fromphotomultiplier tubes 86 begins. Under computer control, each channel 90is opened for signal storage at a predetermined moment after thebeginning of the light impulse. Signals can be stored during thecharging of capacitors within spectral excitation source 52. Multiplexor88 and A/D convertor 98, under the control of central controller 26,read channels 90 and transmit data to computer 100. The signals areprocessed by computer 100 to provide a readout of the constituentelements in the melt and the quantities thereof.

Since some elements are vaporized in the earlier stages of discharge andothers in the later stages, discharge pulses with different parametersmay be included in the sequence of pulses, with each type of dischargeoptimized for special elements. In addition, different spectral linesmay be detected by adjusting the time delay in measurement throughdifferent channels 90, thereby achieving the maximum signal to noiseratio. Switches 96 are opened with predetermined time delays relative tothe beginning of discharge, wherein the time delay may be programmed foreach channel 90 separately to achieve optimal conditions for spectralline emission, either ions or neutral atoms.

The radiation intensity in each channel 90 is measured, and theseintensities are compared with standard values to determine theconcentration of elements in the sample. The elemental concentration ofsample 44 is determined and compared with standard limit values ofconcentrations for types of metals or alloys. The range for eachcritical concentration in sample 44 is determined from the appropriatemetal standard, wherein each metal or alloy has its own criteria ofconcentration of elements. Central controller 26 (FIG. 1) analyzes thedifference between previous, current, and target concentrations ofelements and provides instructions for the control of melting and foradjusting the composition of bath 14 via doser 19 during processing. Avisual imaging system 106 (FIG. 1) allows for the observation of allaspects of metal processing with apparatus 10.

It is understood, of course, that while the form of the invention hereinshown and described constitutes a preferred embodiment of the invention,it is not intended to illustrate all possible forms thereof. It willalso be understood that the words used are words of description ratherthan limitation, and that various changes may be made without departingfrom the spirit and scope of the invention disclosed.

What is claimed is:
 1. A method for processing the chemical compositionof a molten metal bath within a metallurgical vessel, the methodcomprising:obtaining a sample of molten metal from the bath using aprobe; transferring the sample to an atomic emission spectrometer inclose proximity to the vessel by depositing a part of the probecontaining the sample into the atomic emission spectrometer; excitingthe optical atomic emission spectrum of the sample; and analyzing theoptical atomic emission spectrum to determine the elementalconcentration of the sample.
 2. The method of claim 1, wherein obtainingthe sample is accomplished using a robotic arm.
 3. The method of claim1, wherein transferring the sample is accomplished using a robotic arm.4. The method of claim 1, wherein obtaining the sample includes makingseveral horizontal motions with the probe in the bath.
 5. The method ofclaim 1, further comprising joining the deposited part of the probe withan upper chamber housing to form an excitation chamber within the atomicemission spectrometer.
 6. The method of claim 1, wherein exciting theoptical emission spectrum of the sample includes generating a dischargepulse comprising a first part which excites the spectral lines of ionsand a second part which excites the spectral lines of neural atoms. 7.The method of claim 1, wherein exciting the optical emission spectrum ofthe sample includes generating discharge pulses with differentparameters included within a sequence of discharge pulses.
 8. The methodof claim 1, further comprising adjusting the composition of the bathduring processing based on the determined elemental concentration of thesample.
 9. An apparatus for processing the chemical composition of amolten metal bath within a metallurgical vessel, the apparatuscomprising:a probe for obtaining a sample of molten metal from the bath,the probe having a first probe part and a second probe part, wherein thefirst probe part is divisible from the second probe part and the firstprobe part contains the sample; an atomic emission spectrometer locatedin close proximity to the vessel; an excitation chamber formed withinthe atomic emission spectrometer by joining the first probe part with anupper chamber housing; a spectral excitation source operably connectedto the excitation chamber, the spectral excitation source operating toexcite atoms within the sample to emit optical radiation; an opticaldetector operably connected to the excitation chamber to detect andmeasure the optical radiation and convert the radiation intoproportional electrical signals; and a processor operably connected tothe optical detector, the processor receiving the electrical signals anddetermining the elemental concentration of the sample.
 10. The apparatusof claim 9, wherein the probe is disposable.
 11. The apparatus of claim9, wherein the first probe part has inlets which are initially closed bya temperature-destructive material, and then selectively open tointroduce molten metal into the probe after passage of the probe acrossa slag layer to any predetermined depth of the bath.
 12. The apparatusof claim 11, wherein the temperature-destructive material includes cork.13. The apparatus of claim 9, wherein the second probe part includes acover with a planar surface for forming a planar surface on the sample.14. The apparatus of claim 9, further comprising a robotic arm forcontrolling movement of the probe.
 15. The apparatus of claim 14,wherein the robotic arm performs several smooth horizontal motionswithin the bath to obtain the sample.
 16. The apparatus of claim 14,wherein the robotic arm cooperates with a doser to add elements to thebath to adjust the elemental concentration of the bath based on thedetermined elemental concentration of the sample.
 17. The apparatus ofclaim 9, further comprising a table into which the first probe part isdeposited, wherein the table is operable to join the first probe partand the upper chamber housing to form the excitation chamber.
 18. Theapparatus of claim 9, wherein the spectral excitation source generates adischarge pulse comprisinga first part which excites the spectral linesof ions; and a second part which excites the spectral lines of neutralatoms.
 19. The apparatus of claim 9, wherein the spectral excitationsource generates discharge pulses with different parameters within asequence of discharge pulses.